depends what you need the dictionary for
dictionaries are expensive to collate and compile, they are not, as far as Im aware free to download, and if they were I'd be suspicious of the content. there are sme cut down dictionaries available
THE definitive English dictionary is OED (Oxford English Dictionary), other countries such as the US have their own variants.
however it depends what you want to do, they may be other ways round the problem
if its 'just' to check speling you could use somethign like the official scrabble words list (SOWPODS IIRC), it list soem 267,000 words (some plurals, some alternative spellings and some variant/derivations of words).. so say some 150,000 odd words.. but no defintion of the word or guide on how to pronounce it, no synonyms etc......
of course there is
google